Morphology, Textures and Thicknesses of Rubbly Pahoehoe Lavas from the ~117 Ma Sylhet Traps, India

Abstract

The Sylhet Traps and the larger outcrops of the Rajmahal Traps in India constitute an important remnant of a large Continental Flood Basalt (CFB) province related to the Kerguelen hotspot. A composite log of lava flows across the southern margin of the Meghalaya plateau is presented where the Sylhet Traps are best exposed. Lavas in the lower part of stratigraphic sequence (∼480 to 700 m above mean sea level) are compound and contain pahoehoe sheet lobes intruded by thin basaltic dykes constituting a swarm. These lavas record variable degrees of inflation at low effusive rates. In the upper parts, several rubbly pahoehoe lavas are interspersed with sheet lobes. These represent transitional lava types (between pahoehoe and aa) and occur due to steep gradients and/or increased effusive rates. The vesicles from within these lavas impinge and coalesce, indicating that they had time to form and grow in an insulated environment. The vesicle deformation index (VDI) ranges from 0.2 to 0.53, indicating moderate but variable shear strain on the lavas during emplacement. The coexistence of pahoehoe-rubbly pahoehoe lavas is also recorded in the surface outcrops of Rajmahal Traps and the IODP drill cores along the Kerguelen Plateau. It is postulated that the Sylhet lavas represent the distal edge of a lava-shield evolving into a flood basalt setting, dominated by pahoehoe in the lower stratigraphy with few lavas transitioning to rubbly pahoehoe towards the top. Such an architecture is in keeping with other CFB provinces worldwide.